The utilization of halogenated hydrocarbon solvents as carriers for the impregnation of wood with preservatives, dyes, antiblooming agents, fire-retardants and the like has provided the art with a process superior to creosote oil, petroleum oil or liquefied petroleum gas techniques. The halogenated hydrocarbon solvent process is less expensive and more versatile, providing treated wood which is lighter, and therefore generally less expensive to ship. Also, wood treated in this manner has good retention of the additives and may be painted which was more difficult or impossible with older techniques. Such halogenated solvent processes generally comprise soaking wood in a solution of a wood preservative in a relatively low boiling solvent such as methylene chloride or trichloroethylene. Pentachlorophenol is a commonly used preservative. The solution may also contain an antiblooming additive such as a glycol or glycol derivative. The soaking step may be carried out at ambient or somewhat elevated temperature and pressure. When the wood has been sufficiently impregnated by the treating solution, the excess solution is drawn off and the wood is heated to vaporize the solvent in the wood. Solvent removal is completed by air drying. A final treatment of the wood by application of reduced pressure may also be used for this purpose.
The wood treating technique using a halogenated hydrocarbon solvent results in a loss of from about one-half to one and one-half pounds of solvent per cubic foot of wood treated in addition to the additives retained by the wood. Thus, it has been necessary for the operator to "test and add" to his treating tank in order to maintain the desired concentration of treating agents and to make-up solvent lost. In the test and add technique, since many of the preservatives are not readily soluble in the halohydrocarbon solvent and preservatives such as pentachlorophenol are soluble only to about 10 percent by weight in the solvent, the operator must recirculate large quantities of solvent through a solid bed of the preservative to his storage tank. The present invention provides a composition and method for replenishing the depleted treating solution by the continuous or intermittent addition of the liquid concentrate, thereby maintaining the concentration of the treating agents and replacing lost solvent at the same time.